This invention relates to externally ornamental parts which have fine color tone similar to that of gold and which exhibit corrosion resistance and wear resistance properties.
Externally ornamental parts for timepieces and personal effects are required to have fine color tone which is an ornamental factor and at the same time to exhibit corrosion resistance which is a functional factor. In order to produce such externally ornamental parts, gold has been conveniently employed for many years. Gold is employed for the above described purpose after gold material has been suitably processed as it is or in the form of a gold plating layer deposited on other metals. When an externally ornamental part is required to have fine color tone, but not required to exhibit corrosion resistance, desired ornamental effect such as golden color tone can be attained by a gold layer having a thickness less than 1 micron. However, since timepiece cases and bands, and personal effects are required to have high corrosion resistance against sweat, water vapor and the like, and also wear resistance property to ensure a long service life for the parts and articles, the plated gold is required to have a thickness in excess of 10 microns. Since gold is a quite precious metal, the application of the precious metal to externally ornamental parts or articles has been subjected to limitation. Therefore, development of golden color coating layers which are less expensive and have the requirements called for externally ornamental parts or articles referred to hereinabove has been longed for many years. In order to comply with such desire, the so-called "Physical Vapor Deposition" ion plating or sputtering and "Chemical Vapor Deposition" have been developed. By the utilization of these processes, it has been made possible to obtain golden color coatings of TiN and TaN which can satisfy the requirements such as less expense and excellent surface properties called for the coatings of externally ornamental parts or articles. However, although the golden color coatings formed by these processes apparently exhibit a golden color, surface properties, the golden color of the thus formed golden color coatings is substantially different from that of natural gold, when compared with the inherent fine color tone of natural gold. Therefore, the golden color coatings can be applied to only a limited variety of externally ornamental parts or articles. Especially, as the color tone for timepiece cases and bands, color tones such as those of gold alloys are strongly longed for. Although TiN and TaN coatings exhibit excellent corrosion resistance and wear resistance properties, the application of such metal coatings to timepiece cases and bands is subjected to limitation. Timepiece cases and bands are in many cases designed by various combinations of mirror finish and coarse finish. However, mirror finish and coarse finish produce different color tones on the same coating particularly the color tone produced by the coarse finish is substantially different from those of natural gold and gold alloys. According to the lack of color tone balance in a mirror finish and coarse finish, it is frequently difficult to apply TiN and TaN coatings to timepiece cases and bands.